This invention relates generally to radar processing systems, and more particularly to pulse compression systems which are doppler tolerant.
It is well known in the art of pulse radar systems that in order to obtain a good detection capability against a background of noise, a pulse with a large energy content must be transmitted. This larger energy content may be obtained by either transmitting a pulse with a large peak power and/or with a long pulse duration. If the pulse width is limited to small values because of the desire to obtain good range accuracy or resolution, the required energy pulse must be obtained with a large peak power. However, in many applications it is not possible to obtain a peak power as large as one might desire because of voltage-peak limitations somewhere in the system. In such a peak-power-limited radar system, the required energy can be obtained only by transmitting a longer pulse. In order to retain radar resolution (range) when transmitting a long pulse with a high average power content, pulse compression techniques are employed. The use of such pulse compression techniques permit the transmitted pulse to be made as long as desired while retaining an optimum range resolution.
Although there are a wide variety of pulse compression techniques available today, those techniques utilizing either phase or frequency coding are the most energy-efficient. However, with the exception of the Frank phase code (actually a variation of frequency coding) all phase-based techniques are very doppler intolerant. This doppler intolerance arises because of target motion toward or away from the radar system. Such motion tends to change the phasing of the transmitted code and tends to prevent optimum compression unless the system is doppler corrected utilizing multiple decoders. In addition, phase coding and decoding systems require receiver bandwidths much wider than the reciprocal of the compressed pulse length in order to provide low-range time sidelobes. By way of information, the doppler tolerance of a pulse compression technique referred to above is measured by the loss in energy transfer efficiency with doppler shift.
However, it has been found that frequency coding techniques utilizing linear or an approximation to linear frequency modulation are very doppler tolerant. This doppler tolerance is the result of the fact that a doppler shift on any echo from a frequency modulated pulse will simply translate all of the frequency components of the pulse by about the same relative amount in the same direction. As a consequence, all of the frequencies within the radar pass band will still exit from the dispersive delay line at the same relative time to form a short pulse. However, the output pulse will occur at an absolute time different from that which would have resulted in the absence of doppler. The later effect is called range-doppler-coupling.
The major drawback to the use of frequency coding is that the pulse compression techniques presently available limit the pulse compression ratios that can be achieved and produce unnecessarily high sidelobes. In fact, when the dispersive delay lines, i.e., where the delay is proportional to frequency, are employed to compress such long frequency modulated pulses, the energy loss becomes quite significant for larger frequency dispersions. Such compressors produce (sin x)/x time functions without amplitude weighting. The sidelobes present in the (sin x)/x compressor time function can, of course, be reduced via conventional amplitude weighting of the input waveform, but such weighting broadens the mainlobe and reduces the response to targets.